We are continuing our studies of cocarcinogens and, with an expanded list of these compounds, still find that every one tested has proven to be an inhibitor of DNA repair replication. The aqueous phase from whole cigarette smoke condensate as well as the neutral fraction previously reported have been shown to inhibit repair, a finding consistent with their reported cocarcinogenic activities. Attempts to isolate the cocarcinogenic activity from the aqueous phase suggest that there may be more than one repair inhibitor in this fraction. In hope of accelerating research in this area, we have suggested the use of a DNA repair assay as a preliminary screen for cocarcinogens rather than the classical assay, which requires waiting periods of weeks or months for the appearance of tumors. The list of steroids which inhibit repair has also been expanded. Such compounds are being studied from two different standpoints. First, the potentially harmful nature of drugs which inhibit repair has been stressed. The 17 alpha-ethynyl steroid derivative commonly found in birth control pills have also been found to be repair inhibitors. However, blood samples from women taking the pills have been tested without finding any evidence of a decreased repair capability. Additional experiments are planned with cells from the target tissues for these drugs. Our earlier experiments indicated that repair inhibitors could be useful in the sensitization of tumors toward alkylating agents and X- rays. A number of steroids are currently being studied to determine if their target tissue selectivity extends to their property of DNA repair inhibition. If so, they could be useful in the selective sensitization of certain types of tissue toward alkylating agents and X-rays. Several clinical trials reported in the literature have used steroids to enhance the effects of alkylating agents and X-rays. Although other hormonal effects cannot be excluded, serious consideration should be given to the role of DNA repair inhibition when repair inhibiting drugs are used in conjunction with X-rays or alkylating agents.